Lazy<T>

Lazy.swift

class Lazy<T> {
    private(set) var  created: Bool = false
    private(set) lazy var value: T = {
        created = true
        return builder()
    }()
    
    private var builder: () -> T
    
    init(_ builder: @escaping () -> T) {
        self.builder = builder
    }
    
    func map<U>(_ closure: @escaping (T) -> U) -> Lazy<U> {
        return .init { closure(self.value) }
    }
    func flatMap<U>(_ closure: @escaping (T) -> Lazy<U>) -> Lazy<U> {
        return .init { closure(self.value).value }
    }
}


// MARK: - Usage

let a = Lazy { 42 }
    .map { $0 + 10 }
    .flatMap { value in Lazy<String>({ String(value) }) }

a.created // false
let value = a.value //nothing is evaluated until this line
a.created // true

print(value)